PW-Sat is a CubeSat mission designed and developed in the Institute of Radioelectronics at the Warsaw University of Technology (Politechnika Warszawska = PW), Warsaw, Poland. The main objective of the project is education of students in the preparation, construction and use of artificial satellites. The mission objectives are: 1)2)3)4)5)6)7)8)9)

1) System test of a flexible solar generation system

2) Demonstration of a satellite deorbit structure, which will increase the drag of the system at mission EOL (End of Life), resulting in a more rapid descent from orbit.

3) Testing of the satellite communication system, collaboratively developed by the Technical University of Warsaw, the Gdynia Maritime Academy, and the Center for Astronomy at the Nicolaus Copernicus Academy of Sciences, Warsaw.

Figure 1: Photo of the PW-Sat CubeSat (image credit: PW)

Spacecraft:

PW-Sat is a 1U CubeSat form factor with a size of 10 cm x 10 10 cm and a mass of 1 kg. The platform consists of four subsystems, these are:

• Mechanical structure consisting of six aluminum parts for convenient integration of other satellite systems

• C&CS (Communications & Control Subsystem) that consists of digital control logic for performing the required satellite actions and radio circuits for communication with the ground stations using low bit rate transmission.

• AP (Access Port). This subsystem provides a connection for the EGSE (Electrical Ground Support Equipment), to enable external ground control over satellite system. Additionally, AP holds a battery to energize the satellite subsystems when the satellite is in the orbital shadow phase.

Legend to Figure 2: The green and violet lines are power lines, the blue ones are data lines.

Structure: The spacecraft structure is comprised of four machined aluminum 7075-T651 frames fixed together by eight M3 flat head machine steel screws. All aluminum parts are hard anodized to protect the satellite rails from cold welding during the contact with P-POD. The side walls are covered with solar cells.

There is no stabilization system foreseen in the current version of the platform. All subsystems are mounted in stack, the electrical connections between them are realized using a 40 pin bus, stack-through type. Such modular construction of the satellite platform assures an easy and convenient integration of all satellite subsystems and allows a simple replacement and adding of new ones. The mechanical configuration of the platform is shown in Figure 3. The platform allows to accommodate a payload with a mass up to 300 g and a volume of up to 60% of the CubeSat volume.

PSU (Power Supply Unit): PSU supplies the satellite with conditioned power from two sources: solar panels mounted on satellite walls, and a battery providing power for the satellite during the eclipse phase of the orbit. One of main goals in the PSU design was to achieve stability of the subsystem by making it as simple as possible with minimal use of any additional control logic. Therefore, the PSU is fully autonomous and independent from rest of the satellite subsystems. The PSU configuration is presented in Figure 4, it can be divided into five basic functional blocks that are spread on several printed circuit boards:

Both output voltages are protected against latch-up instances. All converters are heavily de-rated, mainly due to heat dissipation issues. The converters and their circuits transferring power from solar panels, are calculated to work with both, low and high efficiency solar panels.

The maximum power generation is ~2 W (with triple junction GaAs solar cells, 27% efficiency). The battery (MP144350 of Saft) has a capacity of 2.6 Ah. The battery is integrated with miniature printed circuit board that contains a charge controller circuits. The charge controller cuts the battery off when its voltage drops to 2.5 V to prevent overdischarge. Similarly, when the battery voltage is above 4.4 V, the battery is cut off to prevent overcharge and leading destruction of it. Additionally, the charge controller allows the battery to be loaded with a maximum current of 3 A.

Figure 5: Photo of the main board assembly of the PSU (image credit: PW)

C&CS (Communications and Control Subsystem): The C&CS supports two tasks:

• Communications with the ground stations, realizing two communications channels – transmission of the CW beacon signal using Morse code, and transmission and reception of APRS (Automatic Packet Response System) frames using AFSK modulation, to receive telecommands and send telemetry and payload data

• Monitoring and control of all satellite systems - execution of received telecommands and preparation of the telemetry data.

The system consists of two RF modules: the beacon transmitter and the APRS transceiver, and the microprocessor controller. Both RF modules are almost identical, consisting of a CC1000 transceiver and power amplifiers in the transmission part (medium power amplifier AH117 and high power amplifier M68710H). The power of the transmitted signal is 20 dBm for the CW beacon transmission and 30 dBm for the APRS transmission using AFSK modulation.

Communications with the ground stations is realized using two separate frequencies in the UHF (437 MHz) band; and beacon signal is transmitted continuously, while the APRS data packets are transmitted in burst mode (each 30-60 seconds, depending on the available power supply budget), with data rate of 1200 bit/s.

The APRS transceiver can also be switched to repeater mode of operation, allowing a realization of communications via satellite for the radio amateur community. Both RF modules are connected to separate antennas, and switches are used to allow the use the same antenna for transmission and reception of radio signal. It is assumed that also the beacon transmitter can operate in receiving mode, allowing to receive telecommands in emergency cases. The generation of CW and AFSK modulated signals, as well as preparation of APRS frames for the transmission and their decoding for reception, are realized by software of the microprocessor controller. Since the CC1000 module is specialized only for FSK modulation, so for transmission of AFSK signal, the controller generates a signal with a frequency of 1200 or 2200 Hz, depending on the data bit value that is sent to the CC1000 FSK modulator. Similarly for the signal reception frequency of the FSK demodulated signal is measured to obtain the received data.

Figure 6: Block diagram of C&CS (image credit: PW)

The system can by remotely configured; it is possible to change the modes of the transmitter, type of modulation, operating frequency, and RF power. Communications realized by the APRS transceiver is compatible with radio amateur standard AX25, APRS, GENSO (Global Educational Network for Satellite Operations), and the DGSS (Distributed Ground Station System); hence, global communications coverage with satellite can be assured, using existing radio amateur ground stations.

Launch: The PW-Sat CubeSat was launched on Feb. 13, 2012 as a secondary payload on the maiden flight of the Vega launch vehicle of ASI and ESA. The launch site was Kourou in French Guiana. 11)12)13)

The multiple payload launch encompasses a primary payload of 400 kg called LARES (LAser RElativity Satellite), and CubeSats (educational payloads) as secondary payloads, whose launch is sponsored by ESA. The free launch of CubeSats was offered by the ESA Education Office in Oct. 2007 (Announcement Opportunity) in cooperation with the Vega program. 14)

CubeSat passenger payloads: Although ESA's Education Office is providing 9 CubeSat positions on the maiden flight of Vega, only 7 CubeSats are confirmed as of December 2011 (Ref. 15). Not all universities that were preselected for the launch opportunity in June 2008, were able to deliver their CubeSat and the requested documentation. Other CubeSat projects, like SwissCube and HiNCube, decided to be launched on commercial flights.

• Xatcobeo (a collaboration of the University of Vigo and INTA, Spain): a mission to demonstrate software-defined radio and solar panel deployment

• e-st@r (Politecnico di Torino, Italy): demonstration of an active 3-axis Attitude Determination and Control system including an inertial measurement unit

• Goliat (University of Bucharest, Romania): imaging of the Earth surface using a digital camera and in-situ measurement of radiation dose and micrometeoroid flux

• PW-Sat (Warsaw University of Technology, Poland): a mission to test a deployable atmospheric drag augmentation device for de-orbiting CubeSats

• MaSat-1 (Budapest University of Technology and Economics, Hungary): a mission to demonstrate various spacecraft avionics, including a power conditioning system, transceiver and on-board data handling.

• UniCubeSat GG (Universitá di Roma ‘La Sapienza’, Italy): the main mission payload concerns the study of the gravity gradient (GG) enhanced by the presence of a deployable boom.

• March 2012: PW-Sat is operational and telemetry data are regularly received at the ground station in Warsaw, Poland. However, the satellite’s power consumption is a little higher than expected, resulting in a slow power drain. Commands have been sent to increase the interval between the telemetry downlinks and to delay, until the power budget is corrected, the release of the deployable drag augmentation device. This device is meant to accelerate the satellite’s deorbiting operations upon conclusion of the mission. 18)

• PW-Sat was heard loud and clear only a few hours after the launch. The signals have been heard both by the team’s ground station and by several radio amateurs from the UK, India, Germany and the Netherlands. The satellite is confirmed to be up and running, and its initial operations are commencing. 19)

Payload/experiment complement: (LEONIDAS, GADGET, OBC)

The payload/experiment complement consists of three parts:

• LEONIDAS - unfolded sail mounted on the skeleton made of the shape-memory material, allowing to test the deorbit technology demonstrator using deployable structure.

Note: The sail experiment design changed over the years after unfolding tests were conducted with various sail shape designs. The final version is a tail-like design which unfolds to a length of ~2 m.

• GADGET - APRS repeater on satellite, working in UHF (437 MHz frequency) band, allowing to test the data gathering by a distributed ground network. After redefinition of the system architecture concept, this subsystem has been incorporated to the C&CS of the platform.

• OBC (On-Board Computer): OBC was considered for the initial concept of PW-Sat as an additional payload, allowing to test its behavior in space. The computer is foreseen as an additional OBC system (OBDH-Node) for the ESEO (European Student Earth Orbiter)microsatellite mission.

The information compiled and edited in this article was provided byHerbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.